An electrostatic paint spray system is a highly efficient technology for the application of paint to specific work pieces. Negatively charged atomized paint particles and a grounded work piece create an electrostatic field that draws the paint particle to the work piece, minimizing overspray.
For this technology, an ionizing electrode, typically located at the paint gun atomizer tip, causes paint particles to pick up additional electrons and become negatively charged. As the coating is deposited on the work piece, the charge dissipates through the ground and returns to the power supply, completing the circuit. The electrostatic field influences the path of the paint particles. Because the charged particles are attracted to the grounded workpiece, overspray is significantly reduced. Paint particles that pass a workpiece can be attracted to and deposited on the back of the piece.
The transfer efficiency is the percent of sprayed paint that is applied to the workpiece. Paint that is not applied to a work piece is captured in the paint spray booth's emission control system and ultimately disposed as waste. The typical transfer efficiency for an electrostatic paint spray systems is 75%.
A potential drawback to electrostatic finishing, particularly for coating complicated surfaces, is the Faraday cage effect: a tendency for charged coating particles to deposit around entrances of cavities. The Faraday cage effect allows electric charges on a conductor to reside on the outer surface of the conductor. In the case of coating complicated surfaces, the electric charge resides on the entrances of cavities. High particle momentum can help overcome Faraday cage effects, since particles with greater momentum (larger particles or particles traveling at higher speeds) are influenced less by the electrostatic forces. However, high particle momentum also lowers efficiency.
Electrostatic paint equipment is available in three basic types: air atomized, airless, and rotating discs and bells. High-speed discs atomize the coating more finely than air atomization and direct more paint to the target. This technology is particularly efficient for the application of difficult to disperse, high-solids paints. However, the Faraday cage effect is generally greater with rotary atomizers than with air or airless types. Rotary atomizers, therefore, may not provide adequate coverage for complicated surfaces.
Electrostatic paint spray systems operate at high voltages (30 to 150 kV). Typical operation is to allow the system to go unused in order to dissipate energy prior to docking the applicator for paint refill, which extends the life of the valves in the paint gun. To more quickly dissipate energy in the system, it is necessary to provide a grounding dissipation unit in the system, preferably near the energized paint. All items in the work area must be grounded, including the operators, the paint booth, the application equipment (unless applying conductive coatings), and conveyors. Ungrounded items should be removed from the work area.